Signaling pathways that regulate self-renewal and differentiation contribute to the cellular heterogeneity within tumors. The varying states of self-renewal and differentiation are evidenced by tumor subpopulations and individual tumor cells that exhibit disparate levels of in vivo tumorigenicity and in vitro clonogenicity. See Lobo et al., Annu. Rev. Cell Dev. Biol., 2007 23: 675-699; Reya et al, Nature, 2001, 414: 105-111. The development of new tumor models has begun to enable the characterization of tumor heterogeneity at cellular levels. Implants of solid tumors in immunocompromised mice exhibit a rich architecture that reflects the histology of the original sample but that is not recapitulated in xenografts from cell lines cultured in serum. The culturing of cancer cells in defined serum-free media and/or three-dimensional matrices preserves the physiological characteristics of the cells more than culturing in media with serum (Lee et al., Cancer Cell, 2006, 9: 391-403). Fluorescence-activated cell sorting (FACS) of cells from tumors, xenografts, and cell lines has facilitated the molecular characterization of specific tumor sub-populations.
In many tumors, cells defined by specific surface markers form tumors more efficiently than other cells in the same tumor. These cells are alternately referred to as multipotent tumor-initiating cells, cancer stem cells, tumor-initiating cells, and cancer-initiating cells. Tumor-initiating cells were first identified in the hematopoietic system (Bonnet & Dick, Nat. Med., 1997, 3(7): 730-737) and have since been identified in solid tumors, including tumors of the brain, breast, colon, head and neck, lung, melanoma, pancreas, and prostate. See Visvader & Lindeman, Nat. Rev. Cancer, 2008, 8: 755-768 and reference cited therein. In a particular tumor type, the same set of cell surface markers can be used to isolate tumor-initiating cells from fresh tumor samples, xenografts, and cell lines. See e.g., Al-Hajj et al., Proc. Natl. Acad. Sci. USA, 2003, 100: 3983-3988; Filmore & Kuperwasser, Breast Cancer Res., 2008, 10: R25; Hermann et al., Cell Stem Cell, 2007, 1: 313-323. Matsui et al., Blood, 2004, 103: 2332-2336. CD44, a marker of tumor-initiating cells in several tumor types, was recently shown to have a direct role in tumorigenesis and to be repressed by p53 (Godar et al., Cell, 2008, 134: 62-73).
Tumor-initiating cells show resistance to standard therapies. For example, tumor-initiating cells were highly enriched in samples from breast cancer patients that had received chemotherapy, suggesting an explanation for disease relapse following treatment (Yu et al., Cell, 2007, 131: 1109-1123). Similarly, CD133+ tumor-initiating cells in glioblastoma were resistant to irradiation that eradicated the more prevalent CD133− cells (Bao et al., Cancer Res., 2006, 68: 6043-6048). Thus, in the context of therapy, eliminating tumor-initiating cells might require targeting mechanisms other than those used to target the bulk of the tumor.
To develop treatments that significantly increase long-term patient survival in cancer, tumor-initiating cells responsible for tumor recurrence and metastasis represent an important therapeutic target for this disease. To meet this need, the present invention provides isolated and enriched populations of tumor-initiating cells that can be used to test the efficacy of new and existing cancer drugs.